1. Field of the Invention
This invention relates to an apparatus for extracting hot gas samples from a reaction vessel, which apparatus comprises at least one extracting probe having a gas-conducting heated inner tube, a cooled outer shell and heat insulation between the inner tube and the outer shell, and also comprises a filter, which succeeds the extracting probe and is composed of a filter cup and a filter plug, which is detachably inserted in the filter cup.
2. Description of the Prior Art
For a control of a reaction in dependence on the composition of hot gases produced by such reaction it is necessary to extract gas samples from the reaction vessel. Owing to the high gas temperatures the extracting probes used for that operation are provided with a cooled shell. But the action of such cooled shell may result in a temperature drop below the dew point temperature so that condensate may undesirably be formed in the inner tube of the extraction probe. Together with the dust content of the gas samples the condensate forms a sludge, which may deposit on the inner tube of the extraction probe and may constrict the flow area. In order to avoid a temperature drop below the dew point temperature it has already been proposed (EP-A-0 243 570) to provide the inner tube with heating means and to shield the inner tube from the cooled shell by heat insulation so that the otherwise occurring action of the cooled shell on the temperature of the inside surface of the inner tube will be prevented and the temperature of the inner tube can be kept in a desired range regardless of the temperature of the cooled shell. The dust which has been extracted with the gas sample must be separated from the gas sample by a filter, which succeeds the extracting probe and in which a temperature drop below the dew point temperature must also be avoided so that the filter will not be clogged by the dust which would be formed. Although heating means may be used for that purpose, a relatively expensive maintenance will be required in that case because the filter cup of the filter will be filled by the retained sludge and must be cleaned in regular intervals of time in that the filter plug is removed from the filter cup and is purged with compressed air and the dust which has been retained by the filter is manually removed from the filter cup.
Gas samples should be extracted from the reaction vessel also during such maintenance operations. For that purpose it is also known (EP-A-0 243 569) to provide two extracting probes, which are connected in parallel and through which the gas samples are extracted from the reaction vessel in alternation so that the required maintenance work can be performed at that extracting probe which is not used to extract gas at a time. In order to reduce the maintenance expenditure, more gas is extracted from the reaction vessel than is required for analysis and the surplus quantity of gas is blown through the respective other probe in a direction which is opposite to the extracting direction so that the filter associated with said other probe will be purged. But in spite of the fact that the filters are regularly purged in a direction which is opposite to the extracting direction, the dust which has been retained by the filter must be removed by hand from the filter cups because the repeated deflection of the purging streams will result in a formation of dead spaces in which dust can deposit.
In order to reduce the larger additional expenditure which is required for the connection of probes to the reaction vessel where two extraction probes are employed, it is also known (FR-A-2 363 792) to combine the extracting probes with a common air-cooled housing in a unit of construction. But that arrangement cannot reduce the expenditure involved in the maintenance work.
In connection with an extracting probe it is known (EP-A-0 219 188) to provide a filter cup which is constituted by a diametrically enlarged, coaxial extension of the inner tube of the extracting probe, so that the resulting decrease of the velocity of flow will promote the settling of the dust in the filter cup. But that feature does not eliminate the need for a manual cleaning of the filter cup even when the filter is purged in a direction which is opposite to the extracting direction. This is due to the fact that the purging air flowing through the filter plug can be used to substantially clean only the filter plug but cannot be used to clean the filter cup.